The present invention relates to electrical power systems and in particular, to alternative energy systems including, but not limited to, wind turbine, water turbine, steam and solar powered photovoltaic systems that supply power for residential, commercial, or industrial use. Nearly all energy systems have a source of direct current (DC) connected to equipment that combine, transform, limit, store, synchronize, and transport the energy for use with electrical appliances.
Photovoltaic (PV) power systems also include a load balancing subsystem comprising DC switching and protection devices, combiner boxes, circuit breakers, disconnect switches, mechanical relays, solid state relays, and contactors, which make or break the flow of current. Combiner boxes aggregate the DC power from the PV module strings and provide a parallel connection point (i.e., a common bus) for the PV strings, with the combiner box providing overcurrent protection and isolation. Combiner boxes are either source combiners or array combiners, with source combiners being located closer to the PV strings and array combiners, or re-combiners, for aggregating outputs from several source combiners into a single circuit.
The present invention focuses in particular to PV systems, which include multiple components, including DC power-producing units called PV modules, mechanical and electrical connections and mountings, and means of regulating or modifying the electrical output. The electrical energy is produced by devices made of, but not limited to, silicates that produce electrical current when exposed to solar radiation (sunlight). When manufactured with electrical conductors that carry the DC current produced the devices are commonly known as solar cells or PV cells. An arrangement of electrically connected PV cells configured is commonly called a PV cell string. One or more PV cell strings are assembled together into a PV module.
A PV system is comprised of components, including PV modules, which are composed of strings of PV cells that produce electricity from solar radiation. The strings of PV cells are joined to each other (if more than one cell string). The level of DC current and voltage produced is dependent on the number of PV cells, the intensity of solar radiation, and other environmental factors.
The efficiency of a PV module determines the area of a PV module given the same rated output (i.e., an 8% efficient fabric will have twice the area of a 16% efficient fabric to produce the same electrical energy output). A PV module can be directly connected to a load or to other PV modules through a junction box on the module, normally located on the back of the PV module. The connections of the PV cells to each other and to the junction box are by a multiplicity of conductors that are attached to the modules and to other conductors to complete the electrical circuit through the PV module.
Output from several PV modules are often combined to aggregate the current or voltage in a PV array. One common arrangement in PV systems is to connect output from PV modules to high-capacity energy storage units, which relate to conventional DC batteries, to store excess energy for use when solar radiation is insufficient. When alternating current (AC) power is desired, an inverter is used to convert the DC energy from the array into AC energy, such as AC energy suitable for transfer to a power grid.
Recent PV modules are equipped with micro inverters at the output that convert DC to AC at the PV module. Such PV modules are wired in parallel, which produces more output than normal panels, which are wired in series with the output of the series determined by the lowest-performing panel. Micro-inverters work independently so each panel contributes its maximum possible output given the available sunlight.
For safety reasons, PV modules are commonly connected to a DC disconnector device. If several PV modules are generating DC, the output is often connected to a DC combiner box. Some of the generated power is often stored in an energy storage unit comprised of batteries in addition to being connected to electrical transformers. Some of these embodiments use DC to DC converters to raise or lower the voltage before delivering the DC current to an alternating current (AC) power converter (also known as an inverter). Output from an inverter usually connects to an AC distribution unit that transforms, signal conditions, synchronizes, and connects the AC power to transmission lines. Systems embodied with only PV modules with AC micro-inverters omit much of this equipment and deliver the AC power output directly. Some additionally connect with an AC to DC converter to a DC storage unit that provides for a reserve capability when night falls or solar radiation is insufficient, which can be caused by increased load or simply clouds, snow, sleet, or rain.
The present invention also relates generally to conventional circuit breakers and Arc-fault Circuit Interrupters (AFCI) used in photovoltaic (PV) arrays. AFCI and conventional thermal circuit breakers only respond to overloads and short circuits on the load side, so they do not protect against thermal conditions or hot spots that lead to arcing conditions that produce erratic, and often reduced current on the source side. Conventional thermal sensors identify the temperature of objects in proximity, but do not protect against electrical arcing. AFCI typically use the arc-generated noise on the DC system to identify the arc-fault and then mitigate it by de-energizing the PV system. This approach is limiting, as it requires an arc-fault to be present before remediation is possible. Since AFCI isolate the PV source (arcing) from the load, current flow is stopped and the whole PV module string is shut down. This action will quench a series arc, but conversely would cause all energy normally flowing to the load to now be shunted into a parallel arc, should that be the fault mode. Such additional energy flowing into the parallel arc would make the problem worse instead of resolving it. Of these sensors, only a thermal sensor or a visual inspection might be able to find a hot spot, but coverage to detect such a condition is problematical at best.
When photovoltaic cells were first produced, their efficiency was limited and the amount of energy produced was small compared with the power produced by PV cells today. Efficiency of the photovoltaic cells is ever-improving with new techniques, such as multiple layered (3-dimensional) cells developed at Sandia National Laboratory.
At the time of preparing this patent application, the most efficient mass-produced solar modules are reported to have energy density values of up to 16.22 W/ft2 (175 W/m2); sufficient electrical energy to cause several failure modes including hot spots, arc-flash events, series arc-faults along the conduction path, and parallel arc-faults that connect to ground through exposed conducting components.
The energy generated by the PV module, whether used alone or when PV modules are connected in series with each other, can result in localized heating. This heating, if severe, can subsequently result in a localized fire, which may spread to any mounting structure or material (including a building), which may be located under or adjacent to the PV panel. The localized heating may also degrade the conduction path in a manner that when sufficient energy is present, a series arc-fault can be established in the conduction path. Such an arc-fault generates hot plasma and intense heat. Since the supporting energy of the arc-fault is DC, there are no current zero crossings as in AC and the arc does not self-extinguish and continues as long as sufficient energy exists. The intense heat readily causes any supporting structure, buildings, etc. to rapidly catch fire.
Another failure mode of electrical systems is an arc-flash that is caused by defects, oxidation, externally induced energy, etc. According to statistics compiled by CapSchell, Inc. (a Chicago-based research and consulting firm that specializes in preventing workplace injuries and deaths), there are five to ten arc-flash explosions that occur in electric equipment every day, resulting in medical treatment. An arc-flash is a breakdown of the air resulting in an arc, which can occur where there is sufficient voltage in an electrical system and a path to ground, neutral, or another phase. An arc-flash, with a high level of current, can cause substantial damage, fire or injury. The massive energy released in an arcing fault can instantly vaporize metal in the path of the arc, blasting molten metal and expanding plasma outward with extreme force. The result of the violent event can cause destruction of equipment, fire, and injury, not only to the worker, but also to nearby persons.
A series arc-fault results when a junction of an electrical circuit opens intentionally or unintentionally. If the fault is a ground fault, the energy from the conduction path conducts to external frame and supporting structure. The intense heat generated can result in the supporting structure, buildings, etc. to rapidly catch fire. Additionally, the parallel arc-fault causes the ground path to become electrically energized, adding an additional shock hazard. Smoke and fire created by all of the PV module faults cause severe difficulties for firefighters since the fire source is electrified with sufficient energy to cause injury. To avoid injury, firefighters, called to the scene of a fire caused by a faulty PV module, generally just hose down neighboring structure, and let the arcing modules burn.
All this increased efficiency poses a significant safety problem caused by DC arc-faults within a PV system. For instance, a PV module is rated by its DC-output power under standard test conditions (STC), at 25 degrees Celsius. Typically, today's output ranges from 100 to 320 watts. In the case of a PV module, the intense heat can ignite combustible materials used in the module construction, which quickly spreads to nearby combustible material, such as grass or roofing materials.
In the United States, the authority having jurisdiction (AHJ) review designs and issue permits, before construction can lawfully begin. Electrical installations, which are governed by the NEC, are inspected by the AHJ to ensure compliance with building code, electrical code, and fire safety code. In particular, electrical installation practices must comply with standards set forth within the National Electric Code (NEC), which governs when individual municipalities or states adopt the requirements set forth therein. The NEC is a publically-available, non-patent document that, because of relevance to the state of art of electrical systems, is included in its entirety herein.
The following is a direct quotation of section 690.11 of the 2014 National Electric Code: “Photovoltaic systems with dc source circuits, dc output circuits, or both, operating at a PV system maximum system voltage of 80 volts or greater, shall be protected by a listed (dc) arc-fault circuit interrupter, PV type, or other system components listed to provide equivalent protection. The PV arc-fault protection means shall comply with the following requirements:                (1) The system shall detect and interrupt arcing faults resulting from a failure in the intended continuity of a conductor, connection, module, or other system component in the dc PV source and dc PV output circuits.        (2) The system shall require that the disabled or disconnected equipment be manually restarted.        (3) The system shall have an annunciator that provides a visual indication the circuit interrupter has operated. This indication shall not reset automatically.”        
Furthermore, existing switch mechanisms in PV power systems cannot be automatically reset/reclosed after isolation of a faulted circuit so as to restore a circuit with healthy strings or combiner boxes—as would be desired in combiner boxes that are remote and not easily accessible. In fact, Section 690.11 of the National Electric Code (NEC), cited previously, specifically prohibits automatic reset/reclose actions for isolating an arc-fault in a PV module. In addition, in existing PV power systems that employ conventional switches that cannot be remotely operated, it is not possible to continue to supply power in a PV system while the unsafe condition on a particular PV string is being addressed, such that the PV power system can continue to operate even while the fault is being addressed. This causes undesirable down time.
There is a plethora of publically available documents such as, “American Electricians Handbook” by T. Croft, F. Hartwell, and W. Summers (which is included in its entirety by reference herein), that teach electrical system designs and installations as well as problems related thereto. Other documents are publicly available that teach how to design protection systems, controllers, photon detectors, circuit interrupters, and logic.
For example, improper shutdown of a single PV module generating electricity can potentially create great harm to the PV system by causing a load shift that results in overloading of circuits, which can potentially cause overheating, arcing, and collateral fire damage.
In addition to the problems that can be caused by improper shutdown of a PV module, as solar cells have become more efficient, PV modules can experience DC arc-faults that continue until the solar radiation diminishes as the sun sets. DC arc-faults can occur even when the operating voltages and currents are within normal bounds; such as, but not limited to, deformation of the structure, thermally induced expansion and contraction, or a manufacturing defect. The problem is so serious that the Fire Protection Association modified the 2014 National Electric Code with requirements such as, but not limited to, installers of PV systems must provide Underwriter's Laboratory (UL) listed detection and interruption of electrical faults to prevent death and injuries resulting from electrocution and fires that can quickly engulf and destroy homes, facilities, property, and cause injury and death.
Currently, PV component architects, designers, installers, and maintainers have few options to choose from to comply with the new NEC requirements to prevent unsafe conditions. One option is a micro inverter with costs starting at over $100 U.S. per panel. There are usually 20 solar panel units in a residential solar array, resulting in a potentially $2000 overall cost increase. A second option is a combiner box containing arc-fault detection, which sells for over $1000 U.S. in today's (2014) market. Neither of these options addresses all of the problems of in-module arcing, which can continue until the sun sets or the module self-destructs beyond the ability to sustain the arc. These products do not detect hot spots or arcing in the solar panel and do not eliminate the danger of fire and the human and property hazards that result in liability to installers, manufacturers, and insurance companies. The existing products only shut down entire systems after an arc has occurred in the junction box or in the wiring that goes to the combiner box.
There is a pressing need for an improved AFCI-like mechanism described in detail in the present invention that acts autonomously to reduce the risk of arc-faults happening and, additionally, a control system that employs logic for purposes of clearing faults on unsafe electrical system components, including PV modules and restoring electrical service of healthy components including, but not limited to, PV sub-arrays, healthy PV strings, and healthy PV modules.
It would therefore be desirable to provide a self-protecting autonomous protection system with pre-arc unsafe-condition detection therein that works even when voltages and currents are within normal limits. Further, the protection system would meet the NEC Section 690.11 and other NEC requirements by annunciating unsafe conditions in PV system equipment and associated wiring. The protection system applied to PV modules would provide mitigation before the arc-fault occurs, shutting down the PV module with an unsafe condition; therefore preventing fire damage and human disasters by properly shutting off only the unsafe module in a safe manner and alerting the system owner or consumer for replacement or reinstatement.
The present application teaches a protection system for electrical power systems. The focus herein is on applying the protection system to PV modules wherein defects (including but not limited to hot spots), and faults (including but not limited to arc-faults), are detected and mitigated before the defect or fault can propagate beyond a localized event. This patent describes use of photons (light) to detect unsafe conditions in virtually any electrical system component and teaches actions that isolate and announce an unsafe condition, defect, or fault in a manner that functional PV modules remain operational and that even unfaulted parts of a PV system remain functional.